1. Field of the Invention
This invention relates to substantially dry, precipitated silica-based particulates, and especially to such particulates wherein the particle size ranges from fine micronized particles to unmilled small beads. The invention also relates to a process for the preparation of such precipitated silica-based particulates.
2. Description of the Prior Art
It is known in the art to use silica-based particulates having larger particle sizes as carriers for organic liquids such as liquid organic additives for rubber. It is also known in the art to use silica-based particulates having smaller particle sizes in defoamer applications.
Silica-based particulates are useful as carriers for organic liquids because the organic liquid may be absorbed by the silica and the combination thereafter handled as a free-flowing powder.
Silica-based particulates have varying capacities for organic liquids. The carrying capacity may be regarded as approximately the maximum amount of organic liquid that may be absorbed by the silica such that the resulting mixture retains the characteristics of a free-flowing powder. As the silica-based particulates absorb an organic liquid, the mixture changes from a free-flowing form to one of a semiplastic agglomeration, with an accompanying increase in viscosity. It will be appreciated that the carrying capacity of a silica for a particular organic liquid depends upon the characteristics and properties of both the silica and the liquid.
Silica-based particulates are commonly employed as carriers of liquid chemicals and resins, which are used in producing rubber articles. For example, Hi-Sil 233 silica, which is commercially available from PPG Industries, Incorporated, is used as a carrier product to absorb a wire adhesion promoter resin referred to as HMMM (hexamethoxymethyl melamine). This resin is commercially available as Cyrez CRA 963 from Cytec Industries, Incorporated and is used in the manufacture of tires.
It is known in the art to use resorcinol with resins such as HMMM in rubber applications. For instance, Cyrez CRA 963 generally requires the use of a donor acceptor, such as resorcinol, to crosslink the resin with the silica in a rubber application. However, there are health and safety concerns associated with the use of resorcinol. U.S. Pat. No. 5,321,070 is directed to a mixture including resorcinol and silica which exhibits limited sublimation behavior with regard to resorcinol. This reference states that the problems and health risks associated with resorcinol are due to the presence of sublimating resorcinol. Thus, the reference discloses the use of resorcinol precondensates to limit the sublimation of resorcinol.
Further, resorcinol-free vulcanizable rubber compositions are known. For example, U.S. Pat. No. 5,298,539 discloses vulcanizable rubber compositions containing uncured rubber, a vulcanizing agent and at least one additive selected from the group consisting of derivatives of melamine, acetoguanamine, benzoguanamine, cyclohexylguanamine and glycoluril monomer and oligomers of these monomers.
Another manner of eliminating resorcinol from vulcanizable rubber compositions has relied on the use of alternative co-reactants. U.S. Pat. No. 4,038,220 describes a vulcanizable rubber composition which comprises a rubber, a filler material, N-(substituted oxymethyl)melamine and at least one of α- or β-naphthol. This reference employs the monohydric phenols, α- or β-naphthol, as methylene acceptors in the resin forming reaction during vulcanization in the absence of resorcinol.
The use of resorcinol-formaldehyde resin to replace resorcinol in vulcanizable rubber compositions is also known. A. Peterson, et. al., “Resorcinol Bonding Systems for Steel Cord Adhesion,” Rubber World (August 1984). Further, U.S. Pat. No. 5,891,938 is directed to a vulcanizable rubber composition containing a high imino alkylated triazine resin additive without the use of resorcinol or resorcinol equivalent co-reactants.
Single component resins typically do not require the presence of resorcinol. However, single component resins are generally incompatible with silica particulates. For example, it has been demonstrated that Cyrez CRA 138L (which is commercially available from Cytec Industries, Incorporated) is incompatible with PPG's silica product known commercially as Hi-Sil SC 72C and Rhodia's silica product known commercially as Zeosil 1165. When Cyrez CRA 138L was imbibed in Hi-Sil SC 72C silica and aged, the resin polymerized and formed hard beads. Such hard beads are typically non-dispersible and, therefore, undesirable for use in the process of making rubber products such as tires. In theory, it is believed that the resultant hard beads were due to crosslinking of the single component resin.
As stated above, it is known in the art to use silica-based particulates having smaller particle sizes in defoamer applications. Typically, silica-based particulates used in defoamer applications are hydrophobic, or at least partially hydrophobic precipitated silica. Examples of such hydrophobic and partially hydrophobic precipitated silica particles are disclosed in U.S. Pat. Nos. 6,191,122, 4,377,493 and 4,344,858.
Additionally, silica-based particulates often contain multivalent cations, such as calcium, magnesium and aluminum. Nonlimiting examples of such materials include aluminosilicates such as Zeolex 23A, which contains 9 wt. % Al as Al2O3 available from J.M Huber Corporation; and the calcium silicates available as Hubersorb 600, which contains 26 wt. % calcium as CaO, available from J.M Huber Corporation and Sipernat 880, which contains 6 wt. % calcium as CaO available from Degussa AG.
Oftentimes, the multivalent cations can be leached from the silica-based particulates, leading to undesirable interactions between the multivalent cations and the other components of the system or composition to which the silica-based particulates have been added. As a non-limiting example, fatty acid salts of multivalent cations often become insoluble in aqueous systems.
It would be desirable to develop silica particulates that do not require the presence of resorcinol and/or are substantially free of multivalent cations.